The present invention relates to a thermosetting adhesive composition used in bonding e.g. an electronic part such as a semiconductor chip to a substrate.
In recent years, with development in the semiconductor industry, a number of bonding methods other than conventional solder bonding are proposed for bonding electronic parts such as a semiconductor chip to a substrate. Among these, adhesive compositions are replacing the solder in consideration of cost, bonding temperature and environmental issues.
According to conventional adhesive compositions for bonding electronic parts, a mainstream prescription includes a main agent such as an epoxy resin and a hardener such as an acid anhydride or imidazole compound, and the main agent is hardened through heating (See the JP-A 6-200228 for example.) In such a composition, in order to shorten the hardening time, the hardener is provided in a highly active form of liquid, or a hardening promoter is added as disclosed in the JP-A 2000-129237 for example.
However, when the hardening speed is increased as conventionally by the use of the liquid hardener or the hardening promoter, the hardening speed at room temperatures is also increased. Therefore, if the adhesive composition is provided as a single-liquid, it will have a short pot life (so called usable time, or a period of time for which the composition can be used), and will have to be stored in a frozen state for example, in order to prevent the main agent and the hardener from reacting each other before use. Further, the frozen composition must be allowed to thaw at room temperature upon use. This not only decreases operability but also can cause a problem that moisture in the air is absorbed by the composition and deteriorates composition characteristics during the thawing process. In an attempt to solve this problem, a proposal was made that a micro-grain powder is fixed onto an active group on the hardener surface thereby coating the active group (Japanese Patent Laid-Open No. 9-87364). The proposed method certainly extends the pot life even when the composition is supplied as a single liquid. On the contrary however, fixing the micro-grain powder onto the active group is an extremely time consuming, low operability process, and it is extremely difficult to maintain a reliable fixation of the micro-grain powder onto the active group for a long period of time. Thus, the improvement in the pot life is not satisfactory.
On the other hand, if the composition is provided in the form of a two-liquid type adhesive, use of a hardener which is highly active with the main agent is possible even without freezing since there is no risk for the main agent and the hardener to react prematurely. On the contrary however, the main agent and the hardener must be mixed with each other upon use, at a cost of decreased operability.
As described above, it has been difficult to provide an adhesive composition which is superior in all aspects of hardening speed, operability and storage (pot life).
The present invention is made under the above-described circumstances, with an aim of providing a single-liquid adhesive composition which has a high hardening speed, good operability and a long pot life.
In order to solve the above-mentioned problems, the present invention makes use of the following technical means. Specifically, the present invention provides a single-liquid type adhesive composition comprising a main agent, an imidazole serving as a hardener, and a hardening promoter for increased hardening speed. The imidazole has its surface covered by a thermoplastic resin, and the hardening promoter is provided by a modified imidazole composition with its imino group (xe2x80x94NHxe2x80x94) having the H replaced by a specific reaction retarder group.
An imidazole compound which is not modified has two nitrogen atoms in its 5-member ring, as shown in the chemical formula (1) given below. These portions which contain nitrogen atoms have high reactivity. One of these nitrogen atoms (in the 1 position) is a member of the imino group, while the other nitrogen atom (in the 3 position) has a single bond with a carbon atom and a double bond with another carbon atom. In this composition, the nitrogen atom in the 1 position becomes active when the hydrogen atom as a member of the imino group is removed, whereas the nitrogen atom in the 3 position becomes active when one arm of the double bond is cut. In this case, the nitrogen in the 1 position is more likely to become active than is the nitrogen in the 3 position (because it is easier to remove the hydrogen atom than to cut the double bond.) However, once the atoms become active, the nitrogen in the 1 position has a lower reactivity than the nitrogen in the 3 position. For this reason, activating the nitrogen in the 1 position will not sufficiently promote the intended hardening, i.e. a reaction between the main agent and the hardener.
In order to solve this problem, the present invention makes use, as a hardening promoter, of a modified imidazole compound with its imino group (xe2x80x94NHxe2x80x94) having the H replaced by a specific reaction retarder group. Thus, the reaction of the imidazole compound is mainly due to the nitrogen in the 3 position which has a higher post-activation reactivity. This makes possible to sufficiently promote the hardening reaction between the main agent and the hardener. 
Here, the specific reaction retarding group refers to a functional group which has a stronger connection with nitrogen than hydrogen does, and which calls for greater energy in order to cut its one hand of the double bond between nitrogen and carbon.
Further, according to the adhesive composition offered by the present invention, the hardener has its surface covered by a thermoplastic resin. Therefore, unless the thermoplastic resin becomes molten, the main agent will not react with the hardener. For this reason, even if the adhesive composition is provided as a single liquid, the composition has a remarkably improved pot life at room temperatures. Further, if provided as a single liquid adhesive, there is no need for mixing the main agent with the hardener upon use, nor is there need for freezing at the time of storage. This offers such advantages as superior operability. Still further, coating the surface of the hardener with a thermoplastic resin is easier than fixing the active group with a micro-grain particle. Thus, decrease in operability as a cost of increased pot life is smaller.
According to the present invention, the main agent can be selected from a number of different thermosetting resins such as epoxy resin, polyimide resin, polyurethane resin, urea resin, and phenol resin. Among others however, the epoxy resin is used most preferably.
The epoxy resin may be any of alicyclic, bifunctional glycidyl ether, polyfunctional glycidyl ether, glycidyl ester, and glycidyl amine.
Examples of the alicyclic epoxy resin include alicyclic diepoxy acetal, alicyclic diepoxy adipate, alicyclic diepoxy carboxylate, and vinyl cyclohexene dioxide.
Examples of the bifunctional glycidyl ether epoxy resin include bisphenol A, brominated bisphenol A, hydrogenated bisphenol A, bisphenol F, bisphenol S, bisphenol AF, biphenyl, naphthalene, and fluorene resins.
Examples of the polyfunctional glycidyl ether epoxy resin include phenol novolak, ortho-cresol novolak, DDP novolak, tris (hydroxyphenyl) methane, and tetra phenylol ethane.
Examples of the glycidyl ester epoxy resin include those produced by condensation between a carbonic acid such as phthalic acid derivative or synthetic fatty acid and epichlorohydrin (ECH).
Examples of the glycidyl amine epoxy resin include tetraglycidyldiaminodiphenylmethane (TGDDM), triglycidylisocyanurate (TGIC), hydantoin resin, 1,3-bis (N,N-diglycidyl aminomethyl) cyclohexane (TETRAD-D), aminophenol resin, aniline resin, and toluidine resin.
The epoxy resins listed above may be used individually or in combination. In particular however, abisphenol F resin is used preferably.
Examples of the imidazole compound used as the hardener according to the present invention include 2-heptadecyl imidazole, 2,4-diamino-6-(2xe2x80x2-methyl-imidazolyl-(1xe2x80x2))-ethyl-S-triazine, 2-phenylimidazole, 2-undecylimidazole, 2,4-diamino-6-(2xe2x80x2-undecylimidazolyl)-ethyl-S-triazine, 2-phenyl-4-methyl imidazole, 2-ethyl-4-methyl imidazole, and 2-methyl imidazole.
The thermoplastic resin for covering the imidazole hardener may be selected from a number of known thermoplastic resins, with consideration to the type (temperature range for active reaction) and relationship between the main agent and the hardener. According to the present invention, thermoplastic resin having the melting point between 50-200xc2x0 C. are used preferably. Examples of such thermoplastic resin include polyethylene, polypropylene, polystyrene, polycarbonate, acrylic resin, acrylonitrile-styrene resin, acrylonitrile-butadiene-styrene resin, and polyoxymethylene.
An amount of the imidazole hardener to be added should be determined in consideration to a type of the hardener, a hardening speed to be achieved, pot life and other factors. An appropriate range may be 50-200 weight portion relative to 100 weight portion of the main agent, for example.
On the other hand, the hardening promoter used in the present invention, i.e. a modified imidazole compound, is selected appropriately in accordance with the kind of hardener to work with. Examples include 2-ethyl-4-methyl imidazole, 2-phenyl imidazole, 2-phenyl-4-methyl imidazole, 1-benzyl-2-phenyl imidazole, 1-benzyl-2-methyl imidazole, 1-cyano ethyl-2-methyl imidazole, 1-cyano ethyl-2 ethyl-4-methyl imidazole, and 1-methyl-2-ethyl imidazole, with their respective imino group having its H replaced with a specific reaction retarder group.
The specific reaction retarder group may, for example, a functional group having an epoxy ring, or hydrocarbon radical.
Examples of the functional group having an epoxy ring include a hydrocarbon radical having a pair of mutually adjacent carbon atoms being bridged with an oxygen atom, or having part of its hydrogen atoms replaced with e.g. a functional group such as a halogen group and an amino group. According to the present invention, use is made preferably of those based on the alkyl group having its mutually adjacent carbon atoms bridged with an oxygen atom.
On the other hand, the hydrocarbon radical is preferably provided by an alkyl group.
An amount of the hardening promoter to be added should be determined in consideration to a type of the hardener, a hardening speed to be achieved and other factors. An appropriate range may be 1-50 weight portion relative to 100 weight portion of the main agent, for example.
The adhesive composition according to the present invention may further contain an electrically conductive particle and may serve as an electrically conductive adhesive composition. The electrically conductive particle to be used in this case may be selected, for example, from metal particles of gold, silver, copper, nickel, iron, aluminum, and stainless steel, as well as glass particles and resin particles dressed with a metal coating. The electrically conductive particle may be further coated with an electrically insulating resin such as an epoxy resin, for uniform dissipation in the main agent.
The adhesive composition according to the present invention may further contain an inorganic filler for controlled rate of thermal expansion. The inorganic filler may be selected, for example, from alumina and silica, as well as nitrides such as aluminum nitride and boron nitride. An amount of addition of the inorganic filler ranges from 5-200 weight portion relative to 100 weight portion of the main agent, for example. Addition of the inorganic filler can cause excessive increase in viscosity of the adhesive composition. In order to avoid this, the inorganic filler should preferably have a maximum particle size not greater than 40 xcexcm.
According to the present invention, a coupler maybe added. When a solid hardening promoter is used, the coupler improves dissipation of the hardener and decreases viscosity of the adhesive composition before the adhesive composition is hardened. Further, the coupler increases affinity between organic material and inorganic material. Addition of the coupler, therefore, to the adhesive composition increases bond between the bonding object (e.g. ceramic substrate and silicon substrate) and the main agent (i.e. thermosetting resin) after the adhesive composition is hardened.
The coupler to be used in the adhesive composition according to the present invention may be selected from silane couplers, silicone couplers, and titanate couplers.
The silane couplers are silane derivatives with their monomers not having siloxane bonds yet. Generally, in these couplers there is only one functional group per monomer that determines its chemical or physical characteristics after the polymerization. Examples of the silane couplers include vinyl trichlorosilane, vinyl tris(2-methoxyethoxy)silane, Vinyl-tris(2-methoxyethoxy)silane, gamma-methacryloxy propyl trimethoxysilane, gamma-methacryloxy propyl triethoxysilane, beta-(3,4-epoxy cyclohexyl) ethyl trimethoxysilane, gamma-glycidoxy propyl triethoxysilane, gamma-aminopropyl triethoxysilane, N-phenyl-gamma-aminopropyl trimethoxysilane, gamma-chloropropyl trimethoxysilane, and gamma-mercaptopropyl trimethoxysilane.
The silicone couplers are those whose monomers already have their main chains provided with siloxane bonds. Part of their side chains is modified to have, in general, a plurality of organic reaction groups, organic compatible groups, organic retarder groups and the like per monomer. Since a desired pattern of organic reaction groups, organic compatible groups, organic retarder groups and the like can be introduced to the side chains, the silicone couplers have greater diversity and freedom chemically and/or physically over the silane couplers. Examples of the silicone couplers include polydimethylsiloxane modified by glycyloxy propyl, polyoxylene, alkoxy, or by a combination of these. Having such modified group, the silicone coupler has a relatively high ability required of the coupler, of increasing affinity between organic material and inorganic material. Therefore, by adding a silicone coupler to the adhesive composition, it becomes possible to increase bond between the bonding object and the main agent (thermosetting resin) as after hardening of the adhesive composition.
Examples of the tinatate couplers include isopropyl tri-isostearoyl titanate, isopropyl tridecylbenzene sulfonyl titanate, isopropyl tris(dioctyl pyrophosphate) titanate, tetra-isopropyl bis(dioctyl phosphite) titanate, tetraoctyl bis(ditridecyl phosphite) titanate, tetra(2,2-diaryloxy methyl-1-butyl)bis(di-tridecyl) phosphite titanate, bis(dioctyl pyrophosphate) oxyacetate titanate, bis(dioctyl pyrophosphate) ethylene titanate, isopropyl tri-octanoyl titanate, isopropyl di-methacryl isostearoyl titanate, isopropyl isostearoyl diacryl titanate, isopropyl tri(dioctyl phosphate) titanate, isopropyl tri-cumyl phenyl titanate, and isopropyl tri(N-aminoethyl-aminoethyl) titanate.
The couplers listed above may be used individually or in combination. Preferably, however, use should be made of gamma-mercaptopropyl trimethoxy silane, gamma-glycidoxy propyl triethoxy silane, or glycyloxypropyl-polyoxyethylene-alkoxy-modified polydimethylsiloxane. In order to take full advantage of the addition of these couplers listed above, the coupler should preferably be added at a ratio of 0.1-1.5 weight portion to 100 weight portion of the thermosetting resin, and more preferably within a range of 0.1-7 weight portion.
Additionally, the adhesive composition according to the present invention may include an antifoaming agent and/or viscosity controller. The antifoaming agent may be provided by silicone oil, silica, isoparaffin or higher aliphatic alcohol. An amount of addition should preferably be not greater than 1 weight portion relative to 100 weight portion of the thermosetting resin. On the other hand, the viscosity controller may be provided by a liquid acryl, and an amount of addition should preferably be not greater than 1 weight portion relative to 100 weight portion of the thermosetting resin.